The proposed study will evaluate the plasma and urinary concentrations of five porphyrins, chemicals produced within the mitochondria of human cells, as early biomarkers of autism spectrum disorder (ASD). Mitochondrial dysfunction has been implicated in ASD. Furthermore, early diagnosis and intervention in ASD can improve the child's cognitive performance, language skills, and adaptive behavior. It is the public health goal of this project that the proposed biomarker will help identify children as candidates for ASD assessment, leading to earlier intervention and potentially improved quality of life for these children. The proposed study will use subjects from the Childhood Autism Risks from Genetics and Environment (CHARGE) study based at the University of California at Davis' Center for Children's Environmental Health. Subjects, all between 24 and 60 months of age and enrolled in the CHARGE study, will include a total of 828 children, 356 with ASD, 323 with non-ASD developmental delays in cognitive and/or adaptive function (DD) used as positive controls, and 149 typically developing (TD) children used as normal controls. Aliquots of plasma and/or urine will be obtained when available from frozen specimens maintained by the CHARGE study, and will be tested for the five porphyrins using high pressure liquid chromatography and tandem mass spectrometry (HPLC-MS/MS). The aims of this project are to demonstrate, using a combination of logistic regression and receiver operating characteristic (ROC) analysis, that these porphyrins can be used to identify up to 20-33% of ASD cases with a specificity of 98% compared to TD controls. Similar analyses will be conducted on subsets of ASD cases (those who do and do not meet the criteria for developmental delay), as well as the DD children without ASD. We will include measures of SES, ethnicity and other covariates in our analyses, and evaluate how age and gender influence sensitivity and specificity. As the data and specimens have already been collected, the efficiency of our proposed study would be very high.